PROCESS FOR FRACTIONATING A HYDROCARBONACEOUS CATALYTICALLY CRACKED PRODUCT STREAM AND A FRACTIONATION

COLUMN THEREFORE

The present invention relates to a process for fractionating a hydrocarbonaceous catalytically cracked product stream. It also relates to a fractionation column therefore. In a further aspect the invention relates to a process for catalytic cracking in which such a fractionation process is being used.

Catalytic cracking is a well-known process that is being widely used in many refineries. In catalytic cracking a hydrocarbon feedstock is fed to a riser reactor into which also a cracking catalyst is fed. During the residence time in the riser reactor the hydrocarbon feedstock is being cracked into lighter products. At cracking also some coke is being formed that deposits onto the cracking catalyst to yield spent catalyst. At the top of the riser reactor the product stream is separated from the spent catalyst, and the spent catalyst is then regenerated by burning off the coke using a regenerating gas. The regenerated catalyst is subsequently recycled to the riser reactor. The heat for the catalytic cracking reaction is supplied by the regenerated catalyst. The vaporous product stream of the catalytic cracking process is separated into various fractions, such as C4 ^~ -alkanes and olefins, naphtha, distillate oils and cycle oils in a fractionation column. In the fractionation of the hot vaporous product stream it is desirable to recycle or reflux at least a portion of the oil that collects in the bottom of the fractionator via a suitable heat exchanger to cool the

oil prior to its reintroduction into the fractionator . This technique is commonly referred to as slurry oil pumparound. The oil moving downwardly through the upper portion of the fractionator is highly heated due to contact with the hot vaporous product stream from the FCC riser reactor, which effluent is typically at a temperature in the range from about 450 to about 550 ⁰ C.

In US-A-4776948 it was recognised that the heat exchanger used to cool the slurry oil is subject to intermittent fouling when in service, which fouling causes undesirable reductions in the FCC unit operating capacity. It was proposed to quench the product stream with a cold slurry oil to a temperature below that at which slurry oil will polymerize and/or form coke or coke precursors by allowing a liquid to flow over an inclined baffle plate downward in the column. This specification is silent about the specific problems concerning the inlet of the fractionation column.

The phenomenon of coke-forming material in the transfer line to the main fractionator was recognised in US-A-5258113. As is described in US-A-5258113 refineries tend to maximise the yield on olefins and gasoline and at the same time to use increasingly heavier feeds. However, by striving to these objectives the effluents tend to contain more coke and more reactive materials that tend to form coke. In US-A-5258113 it was found that the coke deposition in the transfer line to the fractionation column was caused by thermal formation of free radicals, which polymerized and laid down coke in the transfer line. As solution it was proposed to add coke-suppressing additives. This has the disadvantage that alien molecules are added to the hydrocarbonaceous feedstock or products.

We now found that coke deposits that close off the inlet to the fractionation column are found to be formed by down-flowing streams in the fractionation column. This is specially the case when a recycle of at least a portion of the oil that collects in the bottom of the fractionation column is used to cool the hot vaporous product stream that enters the column. It has now been found that the building up or clogging of the inlet by coke deposits can be prevented by passing at least a portion of the product stream upward into the fractionation column.

Accordingly, the present invention provides a process for fractionating a hydrocarbonaceous catalytically cracked product stream comprising: a) passing said product stream to a vertical fractionation column through an inlet; b) redirecting a part of the product stream upon entering the fractionation column upwards into the fractionation column by using a baffle; c) separating the product stream into one or more hydrocarbonaceous fractions; and d) discharging the fractions from the fractionation column.

The invention provides that a vapour of product stream is actively passed upwards into the fractionation column. By actively is meant that the amount of product stream and the velocity of that product stream is higher than normally would be when no action is being taken. By this action it is believed that liquid material that drops down from any internals in the fractionation column or the liquid material of the recycle used for cooling and that may comprise coke-forming components is swept from the area around the inlet so that no coke is deposited in this inlet area.

Part of the product stream is redirected upwards into the fractionation column upon entering the fractionation column by using a baffle. The baffle deflects a part of the product stream upwards into the fractionation column such that any liquid material that drips down is removed from the inlet area, whereas the other part of the product stream is passed straight to the middle of the fractionation column so that a good distribution of the vaporous product stream over the cross-section of the fractionation column is obtained.

Accordingly, in a second aspect of this invention, there is provided a fractionation column comprising an inlet for a product stream and one or more outlets for one or more hydrocarbonaceous fractions, wherein the column is further provided with a baffle in front of the location of the inlet for the product stream, the baffle covering part of the cross section of the inlet.

The part of the cross section of the inlet that is covered by the baffle preferably ranges from 20 to 80%, more preferably from 40 to 60%. Even more preferably, about 50% of the cross section of the inlet is covered by the baffle.

Preferably, the part of the product stream is redirected upwards along the inner wall of the fractionation column. However, there are situations where the inner wall cannot be used sufficiently. This is for example the case when the conduit is extended into the fractionation column. This might be the case when the inner wall is not straight or when the conduit is narrowed to reach a preferred velocity at the inlet. In these cases a so-called dummy wall may be added at the end of the pipe. This dummy wall may be a ring added at the end of the pipe and in this way serves as a wall.

This ring has a diameter preferably at least 2 times, more preferably at least 2.5 times the diameter of the conduit .

It is clear that the baffle must be positioned such that there is significant impact of the product stream onto the baffle. Therefore the baffle is suitably at an angle with the direction of the product stream. More preferably, the baffle is substantially perpendicularly positioned vis-a-vis the flow direction of the product stream. By substantially in this context is understood that deviations up to 15° are possible.

It is desirable to reintroduce at least part of the hydrocarbonaceous fraction that collects in the bottom of the fractionation column into the fractionation column. Generally, this hydrocarbonaceous fraction is an oily fraction, furthermore comprising catalyst fines. Preferably, the fraction is cooled before it is reintroduced. More preferably, the fraction is cooled by passing it through a heat exchanger. To reintroduce the fraction, preferably the column comprises an inlet for reintroducing at least part of the hydrocarbonaceous fraction that collects in the bottom of the fractionation column. More preferably, the inlet for reintroducing the hydrocarbonaceous fraction is located higher than the inlet for the product stream.

It will be evident to the skilled person that the fractionation column may be provided with the usual plurality of fractionation internals, in particular trays. Such trays may be of the bubble-cap, sieve, plate, grid tray, packing or other types. The trays provide contact between liquid and vapour such that separation of hydrocarbons into different fractions occurs as the hydrocarbons condense and evaporate on the trays .

The baffle may have several shapes . It is most convenient that the shape of the baffle corresponds with the shape of the inlet. Since it is almost universal that the inlets to fractionation columns are circular it is preferred that the baffle also relates its shape to a circle. Since only part of the product stream needs to be deflected, the baffle preferably has the shape of a circular segment. Even more preferred, the baffle has the shape of a semi-circle. By this shape it is possible to adjust the portion of the product stream that is passed unhindered into the fractionation column as required. For an optimal deflection, the radius of the baffle, having the shape of a semi-circle, is at least as large as the radius of the inlet. Suitably, the radius of the baffle is from 1 to 2 times the radius of the inlet.

The thickness of the baffle depends on the size, on the material the baffle is made from and on the construction details of the fractionation column and the inlet. Preferably, the thickness of the baffle is between 8 and 50 mm, more preferably between 10 and 35 mm, most preferably between 12 and 25 mm. The skilled person will know what material to use to construct the baffle. Preferably, the material of the baffle is the same as the material of the fractionation column. Suitably, a type of stainless steel or carbon steel may be used.

The skilled person may determine what part of the product flow is passed directly into the fractionation column and what part is passed along the inner wall of the fractionation column, suitably by passing this part onto the baffle described. Suitably the part that is fed directly into the centre of the fractionation column ranges from 20 to 80%, preferably from 40 to 60%. More preferably, about 50% of the product stream is passed

directly into the centre of the fractionation column and another 50% against a baffle with subsequent flow along the inner wall of the column.

The velocity with which the product stream impinges upon the baffle at the inlet side may prevent that coke is being formed on the baffle, even when a very heavy feedstock is being used. However, since it is believed that liquid material that drops down from any internals in the fractionation column and that may comprise coke- forming components is swept from the area around the inlet, this liquid material may drip down via the other side of the baffle so that coke may be deposited on that side of the baffle. To prevent this from happening, the baffle is suitably provided with a hole. Through this hole a small portion of the product stream passes through the baffle. In addition to the product stream that already is deflected by the baffle, this gas stream will blow away any coke that may deposit on the baffle. It is even more effective when preferably a second baffle has been provided downstream the hole that also redirects the small portion product stream that comes through that hole along the surface of the baffle. The dimensions and the shape of the hole can be selected by the skilled person, dependent on the heaviness of the feed, on the composition and amount of the product stream and on the conditions in the fractionation column. The hole is suitably circular and has a radius ranging from 0.1 to 0.3 times the radius of the inlet. The dimensions and shape of the second baffle can also be selected by the skilled person, depending on the circumstances. The second baffle suitably has a shape corresponding to the shape of the hole. When circular, the radius of the second baffle is from 1 to 2 times the radius of the

hole. The second baffle is located downstream the hole, at a distance preferably ranging from 0.1 to 10 times the radius of the hole, more preferably at a distance 0.1 to 2 times the radius of the hole. The baffle is arranged in front of the inlet. The distance from the wall of the fractionation column, at which the baffle has been positioned, suitably ranges from 0.1 to 1.0 times the radius of the inlet. Within this range the skilled person is able to adjust the velocity such that an optimal sweeping away of liquid material is ascertained. The baffle extends preferably parallel to the inner wall of the fractionation column. That could mean that the baffle has the same curvature as the fractionation column. It has been found that the velocity of the product stream plays a role in the process of the invention. Evidently, the most preferred velocity is dependent on other conditions in the fractionation column. In a fluid catalytic cracking process the conditions in a main fractionation column include a temperature ranging from

400 to 600 ⁰ C at the inlet of the column and a pressure of 1 to 5 bara. In such conditions the superficial velocity of the product stream at the inlet of the fractionation column is suitably at least 25 m/s, more preferably at least 30 m/s, even more preferably at least 35 m/s.

Superficial velocity is defined as [volume flow of the gas] / [cross sectional area of the pipe] . The lower limits in velocity are to a large extent determined by the economics (lower velocity needs a too large size of the plant), fouling of the inlet pipe and a too long residence time in the inlet pipe. Evidently there are also optimal upper limits to the velocity. These are to a large extent determined by the disturbance of the

fractionation process in the vessel. Suitably, the superficial velocity of the product stream at the inlet of the fractionation column is not more than 80 m/s, preferably not more than 60 m/s. If the product stream velocity at the inlet is not optimal the skilled person may make the inlet either more narrow to increase the velocity, or broaden the inlet to lower the velocity. The narrowing down of the inlet may be achieved by applying a conical device into the conduit that ends at the inlet of the fractionation column.

The velocity of the part of the product stream that is passed along the wall of the fractionation column is preferably at least half the velocity of the product stream at the inlet. More preferably, the velocity is at least the same velocity of the product stream at the inlet. It is evident that the velocity of the product stream has an influence on the velocity and effectiveness of the sweeping effect of the part that is passed along the wall of the column. Another feature that the skilled person has, to influence the velocity of this part of the product stream, is the positioning of the baffle that suitably may be at a distance from the inlet, ranging from 0.1 to 1.0 times the radius of the inlet.

The fractionation process of the present invention can excellently be used in the catalytic cracking of hydrocarbons. Accordingly, in a further aspect, the present invention provides a process for catalytic cracking of a hydrocarbon feedstock comprising feeding the feedstock to a riser reactor into which also cracking catalyst is fed, followed by removing from the top of the riser reactor a hydrocarbonaceous catalytically cracked product stream and spent catalyst; separating the spent catalyst from the catalytically cracked product stream;

regenerating the spent catalyst with a regeneration gas to produce regenerated catalyst; and feeding regenerated catalyst as cracking catalyst to the riser reactor; wherein the catalytically cracked product stream is fractionated in a process comprising: a) passing said product stream to a vertical fractionation column through an inlet; b) redirecting a part of the product stream upon entering the fractionation column upwards into the fractionation column; c) separating the product stream into one or more hydrocarbonaceous fractions; and d) discharging the fractions from the fractionation column. The invention will be further explained by reference to the following figures.

Figure 1 shows a preferred embodiment of a catalytic cracking process.

Figure 2 shows a preferred embodiment of the inlet of a fractionation column comprising a baffle.

Figure 3 shows a preferred embodiment of the top view of a cross-section of the inlet of the fractionation column.

Figure 1 shows a riser reactor (1) into which a hydrocarbon feedstock is fed via conduit (2) into the riser reactor (1) . Via a conduit (3) hot regenerated catalyst is also fed into the reactor (1) . The mixture of catalyst and hydrocarbon feedstock is passed upwards, while the feedstock is being cracked resulting in spent catalyst and cracked products. The riser reactor (1) debouches into a stripper zone (4) where the cracked products are stripped from the spent catalyst. Usually in the top of the stripper also cyclones are present, wherein the stripped catalyst is separated from the product stream. The stripped catalyst is passed to a regenerator (6) via a conduit (5). Into the

regenerator (6) a regenerator gas, usually an oxygen- containing gas, e.g., air, is fed into the regenerator (6) to burn off any coke present on the spent catalyst. The regenerated catalyst is discharged via the conduit (3) and fed into the riser reactor (1) .

The cracked products are withdrawn from the stripping zone (4) via line (8) and fed into a fractionation column (9) . In the fractionation column the product stream is separated into a number of fractions that are discharged via lines (10), (11), (12) and (13). It will be evident to the skilled person that the number of fractions can be varied. A heavy liquid fraction may be discharged from the fractionation column via line (14). This heavy fraction may contain all the catalyst fines that have been carried over from the reactor and/or the stripper. This stream may be (partly) fed back to the column via line (15). Before entering the fractionation column above the inlet of line (8) the stream is cooled in heat exchanger (16) . Figure 2 shows a more detailed view of the inlet of the fractionation column. It shows a wall (20) of a fractionation column into which a conduit (21) for the supply of hydrocarbonaceous product stream debouches at an inlet (22) . Via one or more supports (23) a semi- circular baffle (24) is provided at a short distance from the inlet (22) . The radius of the baffle is larger than the radius of the conduit (21) . Good results have been obtained when the baffle radius is about 1.5 times bigger. The baffle has been provided with a hole (25) having a radius that may be from one fifth to one twentieth, in particular about one eighth, of the radius of the inlet (22) . Downstream of the hole there is

provided a second smaller baffle (26) to deflect any product stream that passes through the hole (25).

Figure 3 shows a top view of the baffle of Figure 2. The figure shows that the baffle (24) is connected to the wall (20) of the fractionation column via supports (23) . It also shows that the baffle (24) extends parallel to the wall (20) following the same curvature.